xin 



PHYLUM CHORDATA 



89 



from one another by valvular apertures (Fig. 782) which allow of 

 the flow of blood in one direction only, viz. from behind forwards 

 that is, from sinus to auricle, auricle to ventricle, and ventricle to 

 conus. The heart is made of striped muscle of a special kind the 

 only involuntary muscle in the body having this histological 

 character which is particularly thick and strong in the ventricle. 

 It is lined internally by epithelium and covered externally by 

 the visceral layer of the pericardium. 



Springing from the ventricle, or from the conus when that 

 chamber is present, and passing directly forwards in the middle line 

 below the gills, is a large, thick-walled, elastic blood-vessel, the 

 ventral aorta (Figs. 769, B, and 781, v. ao.). At its origin, 

 which may be dilated to form a bulbus aortce, are valves so 



e? at 



FIG. 782. Diagram illustrating the course of the circulation in a Fish. Vessels containing 

 aerated blood red, those containing non-aerated blood blue, lymphatics black. B, capil- 

 laries of the body generally ; E, of the enteric canal ; G, of the gills ; K, of the kidneys ; L, 

 of the liver ; T, of the tail. a. br. a. afferent branchial arteries ; au. auricle ; c. a. conus 

 arteriosus : d. ao. dorsal aorta ; e. br. a. efferent branchial arteries ; h. p. y. hepatic portal 

 vein ; h. v. hepatic vein ; Ic. lacteals ; ly. lymphatics : pr. ci\ v. precaval veins ; r. p. v. renal 

 portal veins ; s. i\ sinus venosus ; v. ventricle ; v. ao. ventral aorta. The arrows show the 

 direction of the current. (From Parker's Elementary Biology.) 



disposed as to allow of the flow of blood in one direction only, 

 viz. from the ventricle into the aorta. It gives off on each side 

 a series of half-hoop-like vessels, the afferent branchial arteries 

 (a. br. a.), one to each gill. These vessels ramify extensively, 

 and their ultimate branches open into a network of microscopic 

 tubes or capillaries (Fig. 782, G), having walls formed of a single 

 layer of epithelial cells, which permeate the connective-tissue layer 

 of the branchial filaments, and have therefore nothing between 

 them and the surrounding water but the epithelium of the 

 filaments. The blood, driven by the contractions of the heart into 

 the ventral aorta, is pumped into these respiratory capillaries, and 

 there exchanges its superfluous carbonic acid for oxygen. It then 

 passes from the capillaries into another set of vessels which join 

 with one another, like the tributaries of a river, into larger and 

 larger trunks^; finally uniting in each gill, into an efferent branchial 

 artery (e. br. a.). The efferent arteries of both sides pass upwards 

 and discharge into a median longitudinal vessel, the dorsal aorta 



